Orthopedic simulator with integral load actuators

ABSTRACT

An orthopedic simulator is provided with a plurality of test stations for each holding a test specimen. A plurality of load actuators are provided, with a respective load actuator for each of the test stations. The load actuators are substantially frictionless and configured to apply loads to the test specimens at the test stations.

RELATED APPLICATIONS

The present application claims priority to Provisional Patent Application 60/760,595 filed Jan. 20, 2006, U.S. patent application Ser. No. 11/332,407, filed Jan. 13, 2006 and U.S. patent application Ser. No. 11/335,974 filed Jan. 20, 2006 the contents of which are incorporated herein, by reference, in their entirety. This application is a continuation-in-part of U.S. patent application Ser. No. 11/332,407, filed Jan. 13, 2006 and continuation-in-part of U.S. patent application Ser. No. 11/335,974 filed Jan. 20, 2006.

FIELD

There is an ever increasing need for testing of orthopedic devices. Such testing may be required for certification of the devices. For example, wear testing of spinal implants are subject to ISO and ASTM standards. In the example of a spinal wear implant, the test procedure defines the relative angular movement between articulating components, and specifies the pattern of the applied force, speed and duration of testing, sample configuration and test environment to be used for the wear testing of total intervertebral spinal disk prostheses. Systems may provide testing with a six degree of freedom for the motions and forces, for example. While the test method focuses on wear testing, additional mechanical tests such as fatigue testing and others can be required. Spinal implants are only one type of orthopedic device. Others include, for example, hip-joint prostheses, knee-joints, etc.

Accuracy in the testing of orthopedic devices can be affected by the mechanisms employed to provide the motions and forces to the orthopedic devices. For example, force actuators typically have some amount of friction, such as that created by seals, that is present during the operation of the actuators. This friction creates some potential error, especially in a multi-station testing environment where the amount of friction may vary from actuator to actuator. The test results on the different test specimens being simultaneously tested may then vary, but not due to any inherent quality of the test specimens, but rather to the differences in the actuators. Furthermore, the accuracy of the loading to provide specific desired loads and forces on the test specimens can also be called into question because of the actuator design.

SUMMARY

There is a need for an orthopedic simulator with actuators for applying motions and forces to a test specimen, such as a spinal implant, in a manner that maximizes performance and accuracy.

The above stated needs and others are met by embodiments of the present invention which provide an integral load actuator comprising a housing, a piston axially slideable within the housing, hydrostatic bearings that float the piston; and hydraulic fluid ports in the piston housing. Hydraulic fluid pressure in the hydraulic fluid ports control substantially frictionless extension and retraction of the piston with respect to the housing.

The earlier stated needs and others are met by embodiments of the present invention which provide an orthopedic simulator, comprising a plurality of test stations for each holding a test specimen; and a plurality of load actuators, with a respective load actuator for each of the test stations, the load actuators being substantially frictionless and configured to apply loads to the test specimens at the test stations.

The earlier stated needs and others are met by embodiments of the present invention which provide a spinal implant wear test machine, comprising a plurality of test stations configured to hold test specimens in position for application of vertical load forces on the test specimens; and a plurality of vertical load actuators positioned relative to the test stations to apply the vertical load forces to the test specimens, each load actuator having a piston that is movable with extremely low linear friction to apply the vertical load forces.

The foregoing and other features, aspects and advantages of the disclosed embodiments will become more apparent from the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front, perspective view of an orthopedic simulator in accordance with certain embodiments of the invention, with an external housing removed for illustrative purposes, and with forces being schematically depicted.

FIG. 2 a is a top view of the orthopedic simulator of FIG. 1; FIG. 2 b is a front view; FIG. 2 c is a bottom view and FIG. 2 d is a side view.

FIG. 3 is a view similar to FIG. 1, illustrating the removability of a specimen containment module.

FIG. 4 depicts an exemplary embodiment of an assembled specimen containment module.

FIG. 5 is an exploded view of the specimen containment module of FIG. 4.

FIG. 6 is a side, partially cross-sectional view of the specimen containment module of FIG. 4.

FIG. 7 is a top view of a base of the specimen containment module of FIG. 4.

FIG. 8 is a schematic depiction of an embodiment of a circulation loop for circulating a temperature control fluid in a temperature control circuit.

FIG. 9 depicts two test stations, with one test station having a specimen containment module releasably attached thereto.

FIG. 10 schematically depicts an exemplary arrangement for circulating bath fluid.

FIG. 11 depicts an embodiment of a specimen containment module in an installed position.

FIG. 12 is a perspective view of the orthopedic simulator of FIG. 1, with an indication of the flexion and extension motion.

FIG. 13 is a cross-sectional view of a portion of a flexion/extension motion linkage in accordance with embodiments of the invention.

FIG. 14 is a perspective view of the orthopedic simulator of FIG. 1, with an indication of the lateral bending motion around an axis of rotation.

FIG. 15 is a rear perspective view of the orthopedic simulator of FIG. 1.

FIG. 16 is a perspective view of the orthopedic simulator of FIG. 1, with an indication of anterior/posterior and lateral translation motions.

FIG. 17 depicts a portion of an x-y slide assembly in accordance with embodiments of the present invention.

FIG. 18 is a perspective view of the x-y slide assembly in accordance with embodiments of the present invention.

FIG. 19 is an exploded view of the x-y slide assembly of FIG. 18.

FIG. 20 is a perspective view of the orthopedic simulator of FIG. 1, with an indication of loading in a vertical direction.

FIG. 21 is a perspective view of an embodiment of an actuator in isolation.

FIG. 22 is a top view of the actuator of FIG. 21.

FIG. 23 is a side view of the actuator of FIG. 21.

FIG. 24 is a cross-sectional view of the actuator of FIG. 21.

FIG. 25 is a perspective view of the orthopedic simulator of FIG. 1, with an indication of the axial rotation linkage and a moment provided at a test specimen.

FIG. 26 is a rear perspective view of the orthopedic simulator of FIG. 1, illustrating an embodiment of a central manifold in accordance with embodiments of the present invention.

FIGS. 27-29 schematically depict different approaches to linkages.

FIG. 30 schematically depicts a nesting order of forces in accordance with embodiments of the present invention.

FIG. 31 shows the required forces for application to a test specimen intended for a lumbar region according to an exemplary set of curves.

FIG. 32 shows the same information as FIG. 31, but for cervical data.

FIG. 33 shows curves for non-sinusoidal input data in accordance with exemplary embodiments of the invention.

FIG. 34 depicts the orthopedic simulator within a housing.

DETAILED DESCRIPTION

The orthopedic simulator of the present invention may be employed, for example, as a spinal implant wear test machine. In such a configuration, the intent of ISO 18192 is satisfied. The orthopedic simulator is designed for accuracy as well as increased speed.

In the following description, it is assumed that the orthopedic simulator is a spinal implant wear test machine, but it should be apparent to those of ordinary skill in the art that this is exemplary only. The features, concepts and designs depicted in the following figures and description may be employed in other types of machines and orthopedic simulators.

The embodiments of the present invention address and solve problems related to the application of accurate and repeatable forces to test specimens. In particular, the invention solves such problems, at least in part, by providing an orthopedic simulator, comprising a plurality of test stations for each holding a test specimen; and a plurality of load actuators, with a respective load actuator for each of the test stations, the load actuators being substantially frictionless and configured to apply loads to the test specimens at the test stations. The problems are also solved, at least in part, by providing an integral load actuator comprising a housing, a piston axially slideable within the housing, hydrostatic bearings that float the piston; and hydraulic fluid ports in the piston housing. Hydraulic fluid pressure in the hydraulic fluid ports control substantially frictionless extension and retraction of the piston with respect to the housing. A still further embodiment also solves the problems, at least in part, by providing a spinal implant wear test machine, comprising a plurality of test stations configured to hold test specimens in position for application of vertical load forces on the test specimens; and a plurality of vertical load actuators positioned relative to the test stations to apply the vertical load forces to the test specimens, each load actuator having a piston that is movable with extremely low linear friction to apply the vertical load forces.

The substantially frictionless extension and retraction of the actuators provides loading of test specimens that is consistent across a plurality of test stations, and is accurate as the forces that are to be applied to the test specimens are not substantially affected by friction effects in the actuators. Other features and characteristics of the actuators provide further advantages.

FIG. 1 depicts an orthopedic simulator 10 for testing of test specimens of orthopedic devices. The orthopedic simulator 10 has a plurality of test stations 12. In the illustrated embodiment, there are six test stations 12 in which specimens are subjected to the forces applied by the machine 10, and a control station 14 that holds a specimen that is not subjected to all of the forces provided at the other test stations 12.

The orthopedic simulator 10 is able to provide forces Fx, Fy, and Fz in the x, y and z directions as depicted in FIG. 1, shown with the x, y and z axes at one of the test stations 12. Additionally, torques may be applied around the x, y and z axes, as depicted. The test specimen is not shown in FIG. 1 so as not to obscure the present invention. In the spinal implant wear testing machine according to certain embodiments of the invention, a specimen containment module is provided that contains fluids in which the test specimen is immersed. Upper and lower adapters 18 (only seen clearly at one of the test stations 12 in which the specimen chamber is removed for illustrative purposes) hold the test specimens between them within the specimen containment module 16.

A linkage 20 provides forces in the x direction with the linkage 22 providing forces in the y direction. Gimbals 24 are connected to the upper adapters 18 and may be moved around the y axis and around the x axis to provide moments around the x and y axes.

Vertical loads, represented by forces along the z axis, are provided by vertical load actuators 26, as shown in FIG. 1. Although different types of actuators may be employed, a friction-free axial actuator is preferable to provide for a friction-free axial/torsion actuation system. The vertical load actuator 26 applies a vertical loading along the z axis through components 28 to the test specimen via the lower adapter 18. In the illustrated embodiment, which will be described in more detail later, the components 28 include an x-y slide table and a load cell.

In is desirable to provide a transmission of drive torque with little deflection related error, having high torsional stiffness. At the same time, low axial stiffness is desirable so that there is little cross-talk onto the vertical loading and so the cross-talk is not seen at the load cell. An axial rotation linkage 30 is coupled to the vertical load actuator 26. The motion of the axial rotation linkage 30 is around the vertical axis z, as depicted in FIG. 1. Although the axial rotation linkage 30 is depicted at the bottom of FIG. 1, it should be apparent to those of skill in the art that the structure depicted in FIG. 1 is suspended vertically so that the axial rotation linkages 30 are free to rotate. This will become more apparent in later-described figures.

FIGS. 2 a-2 d depict alternate views of the orthopedic simulator 10. FIG. 2 a is a top view which best shows the arrangement of the linkages 20 with the gimbals 24. A crosshead 32 is provided, which may also best be seen in FIG. 2 d. FIG. 2 a is a top view, while FIG. 2 b is a front view, FIG. 2 c is a bottom view and FIG. 2 d is a side view.

FIG. 3 depicts a perspective view of the orthopedic simulator of FIG. 1, with a specimen containment module 16 that is remote from the orthopedic simulator 10. The specimen containment modules 16 are releasably attachable to the test station 12. The releasable attachment feature of each of the specimen containment modules 16 enables bench top preparation work on the test specimen to be performed remotely from the environment of the orthopedic simulator 30. This remote loading and preparation capability allows for careful removal and insertion of delicate test specimens. Further, the mounting of one-piece specimens is facilitated with this arrangement. An important consideration is the reduction in the contamination potential created by remotely mounting a specimen within the specimen containment module. The specimen containment module 16 also contains adapters 18 that are designed for flexibility, ease of manufacturing and low cost.

An exemplary embodiment of a specimen containment module 16 is shown in isolation in FIG. 4, and in exploded view in FIG. 5. The specimen containment module contains a base 34 and upper connector 37 that interface to a test station 12 and at which the specimen containment module 16 is releasably attached to the orthopedic simulator 10. A chamber 36, when inserted into the moat 38 in the base 34, forms a fluid container with the base 34. A test specimen 40 is depicted with a lower portion 40 a and an upper portion 40 b. However, certain test specimens may also be one-piece specimens.

Releasable fasteners 42, such as thumb screws, may be employed to releasably attach the specimen containment module 16 to the orthopedic simulator 10. Fluid connections 44 are used to provide fluid as will be described in more detail in the following figures.

FIG. 6 is a side, partially cross-sectional view of the specimen containment module 16 of FIGS. 4 and 5. The test specimen 40 is shown with the upper and lower portions coupled together, as seen in FIG. 6.

FIG. 7 is a top view of the base 34. A specimen mounting platform 46 is provided which includes two pins 48, with one pin piloting and another pin interacting with a slot in the lower adapter 18 a for anti-rotation purposes. Screw holes 50 are depicted and may be employed to provide a specimen hold down function.

The base 34 also includes a recess 52 that is able to interact with a pin 54 on the orthopedic simulator 10. This provides a slidable installation of the specimen containment module 16. A tubing loop 56 is provided within the base to provide a temperature control of the bath in which the test specimen 40 is immersed. As will be described in more detail, a temperature control fluid is circulated through the tubing loop 56 to precisely control the temperature of the bath. The temperature control fluid does not intermix with the bath fluid. A temperature probe 60 provides feedback on the temperature of the bath and can be used to control the temperature control fluid. The signal from the temperature probe 60 is provided as a feedback signal to a heater (not shown in FIG. 7).

Recesses 58 provide for thumb screws or other releasable fasteners to secure the specimen containment module 16 to the orthopedic simulator 10. Bath fluid circulation tubes 62 are used to circulate bath fluid within the fluid container formed by the base 34 and the chamber 36, as will be described in more detail later with respect to FIG. 9.

FIG. 8 depicts a circulation loop for circulating the temperature control fluid in the temperature control circuit. The temperature control fluid is circulated in each of the specimen containment modules 16 through the tubing loops 56, seen in FIG. 7. The tubing loops 56 are connected to a single circulation loop 64 that circulates a temperature control fluid, such as water, through the closed loop system. Although water is an exemplary temperature control fluid, other fluids may be employed as a temperature control medium in different embodiments. The tempered water is circulated through the heat exchangers in each of the baths of the specimen containment modules 16. A heater 66 provides a precise control and circulation of the tempered water. The heater 66 receives temperature signals from the temperature probes 60 and employs this information to control the temperature of the temperature control fluid, and hence, the bath in each of the specimen containment modules 16.

The daisy-chained approach depicted in FIG. 8 produces a very stable temperature in each of the baths at the specimen containment modules 16. In addition to stability, a consistency of temperature from station 12 to station 12 is achieved since the entire circulation loop 64 reaches a stabilized temperature. Also, a single heater may be employed, reducing costs.

FIG. 9 depicts two test stations 12, one of which has a specimen containment module 16 releasably attached thereto. A non-contact level sensor 68, such as those known in the sensing art, are provided on posts 70 near the chamber 36. The height of the non-contact level sensor 68 may be adjusted along the pillar 70 in the direction of arrow 72. This allows the desired fluid height within the chamber 36 to be precisely adjusted. The non-contact level sensor 68 provides its signals to a fill controller 74, schematically indicated as being connected to a non-contact level sensor 68. The fill controller 74, based upon the signals received from the non-contact level sensor 68, determines whether the fluid in the specimen containment module 16 needs to be replenished. The test fluid, such as bovine fluid, for example, may evaporate to some extent, thereby increasing the concentration of the fluid. Distilled water is furnished through a fill tube 75 under the control of the fill controller 74. Only one fill tube 75 is depicted (schematically) in FIG. 9 for illustrative purpose, although in preferred embodiments, each test station 12 has its own fill tube 75. Hence, each the fluid concentration in each of the respective test stations 12 can be individually controlled.

In practice, the non-contact level sensor 68 is positioned to measure the initial height of the bath, providing a fluid level signal to the fill controller 74. The fluid level may be considered a parameter related to the fluid concentration of the bath, since the bath will have an initial concentration at the initial fluid level, prior to any evaporation. When evaporation occurs, the fluid level of the bath will decrease. The concentration of the bath will increase. The decrease in the fluid level, from the initial fluid level, is related to the increase in concentration. In order to restore or maintain the concentration at the desired level, i.e., the initial concentration, the fill controller 74 causes distilled water to re-fill the specimen container until the fluid level of the bath reaches the initial fluid level. This restores the concentration of the bath to its initial level.

Each of the test stations 12 is monitored and the concentration of the fluid maintained in the same manner as described above so as to provide consistency from test station 12 to test station 12. Although embodiments have been described that employ a fluid level as the parameter that is monitored, such embodiments are exemplary only, as other parameters are contemplated, and may also depend on the type of bath. For example, a chemical sensing element may be used in certain embodiments that detects the concentration of certain chemicals in a bath. The exemplary embodiment employs a non-contact level sensor, which is preferred due to its ability to sense the level of the fluid without contaminating the bath. Other types of level sensors, such as contact sensors, are employed in other embodiments of the invention.

An arrangement for the circulation of the bath fluid is depicted in FIG. 10. Unlike the temperature control fluid, individual loops are preferred in order to maintain each test specimen and bath in its own environment. In other words, cross-contamination of wear particles is avoided by providing the individual loops for each specimen module. In certain embodiments, peristaltic pumps 76 are employed for each of the individual loops. A stirring action is provided.

FIG. 11 shows a specimen containment module 16 (without the chamber 36 for illustrative purposes) in an installed position within the orthopedic simulator 10. The specimen containment module is releasably attached at its base 34 to a load cell module 78. The load cell module is designed to accommodate either a single or multi-axis force transducer. In the illustrated embodiment, a single axis transducer is depicted.

FIG. 12 depicts the orthopedic simulator 10 and exemplifies the flexion/extension motion. The linear actuator 20 a of the linkage 20 extends back and forth in an axial manner, causing the connecting link 20 b to translate in an axial direction. This causes the inner gimbals 24 at the test stations 12 to move and rotate around an axis of rotation depicted in FIG. 12. The connecting link 20 b is a solid cross-piece, which provides the advantage of not introducing additive joint error, as the gimbals 24 are each independently connected to the connecting link 20 b. This aspect will be described in more detail with respect to FIGS. 27-29.

Although not shown, the connecting link 20 b and connections to the inner gimbals 24 employ high quality bearings, such as long life needle bearings used at key points. The design insures a long life and low lash, creating an accurate machine for a long term use. The low moving mass linkage depicted maximizes performance and is designed for ease of maintenance.

FIG. 13 depicts a cross-sectional portion of the flexion/extension motion linkage. The inner gimbal 24 is depicted as being connected to the upper specimen adapter 18 b. A stationary bearing housing 80 houses the needle bearings mentioned before. A radial needle bearing 84 is provided, as well as a needle roller thrust bearing 82, which are provided in two places. A tubular shaft 86 permits rotation of the gimbals 24.

A lateral bending motion around the axis of rotation is depicted in FIG. 14. A moving cross-head 32 (also seen in FIGS. 2 a-2 d) is coupled via a connecting link 88 that is moved by linear actuator 90 in an up-and-down motion. This causes the inner gimbals 24 to be pivoted around the axis of rotation.

A rear view of the orthopedic simulator 10 is provided in FIG. 15. The moving cross-head 32 is shown as extending across the orthopedic simulator 10. Also shown in this figure is a central manifold 92, which will be discussed in more detail later. As with the flexion/extension linkages, it is preferred to use long life needle bearings that are of high quality at the key points in the lateral bending motion linkages. These designs ensure long life and low lash, creating an accurate machine for long term use. The low moving mass crosshead assembly maximizes performance. For example, the crosshead assembly 32 may be made of aluminum to provide a very light weight moving mass. In motion, the moving crosshead 32 pivots around the x-axis depicted in FIG. 15.

FIG. 16 shows the orthopedic simulator and depicts the anterior/posterior and lateral translation motions. A translation stage 96 is illustrated in this drawing. The translation stage includes an x-y slide assembly as will be see in the following figures. FIG. 17 depicts a portion of the x-y slide assembly 100 that shows linear slides 102 with a space 104 being provided for springs that produce a biasing force if desired.

FIG. 18 is a perspective view of the x-y slide table 100 constructed in accordance with embodiments of the present invention. FIG. 19 shows the x-y slide table 100 in an exploded view. The x-y slide assembly 100 forms a very compact package, with a very light weight assembly. There is a high torsion and shear capability of assembly with high axial dynamic load ratings for each x-y slide assembly 100. Each slide assembly 100 also has high moment load ratings, due to its efficient design. There is an ultra-low coefficient of linear static and dynamic friction provided by the design. Double-row/side miniature roller bearings reduce or eliminate fretting corrosion. Grease may be provided to assist in the elimination of fretting corrosion and further reduce the coefficient of friction and the start up “stiction.”

The x-y slide assembly 100 of the present invention may incorporate three different modes of operation. These include free-floating to self-center a specimen; a positive axis lock within dynamic range; and an ability to produce a large amount of static shear force, on each axis, for simultaneous shear plane loading of specimens. The x-axis transition plate has a built-in capability to align the upper specimen tooling and the load cell radially.

The x-y slide assembly 100 of the present invention overcomes particular concerns. For example, other such assemblies in orthopedic simulators used ball bearings in the slide design which lend themselves to fretting and skidding when translating. Other advantages of the present invention include the production of simultaneous transverse shear in a compact design, while producing friction-free stage floating, but yet is infinitely lockable within a dynamic range. The lowest inertia assembly for Mz rotation is produced, at all six test stations 12. The design of the x-y slide assembly 100 can withstand a large amount of lbsF in compression. Further, the x-y slide assembly 100 is a translation assembly that can be easily removed from the Fz actuator 26. It also provides a translation assembly that has over-turning moment capability to react moments caused by side loads that are off-centered loading.

In the mechanism of the present invention, the Fx and Fy motions and forces take place in the x-y slide assembly 100, when in the spring-loaded configuration described above. An adjustment system allows an operator to set the amount of force in each of the x and y axes. This is not a controlled degree of freedom, but rather, there is free translation if an external force overcomes the spring setting. For spinal implants that are simple ball-in-socket joints located coincident with the Mx, My and Mz centers of the machine, the spring is not engaged. However, some specimens would generate crosstalk loading into the Fx/Fy or Fz axes. This spring constraining force allows a user to simulate the soft tissue surrounding a specimen, or intentionally sideload an implant to simulate mis-implantation.

The x-y slide assembly 100 includes a lower translation plate 100 and an upper translation plate 112. In certain embodiments, the lower translation plate 110 translates along the x-axis while the upper translation plate 112 translates along the y axis. A base 114 supports the x-y slide assembly and may be mounted on the load cell depicted earlier. Pins 116 are provided and pressed into base 114 and lower translation plate 110. The pins 116 aid in assembly of thee the first mounted slide/rail at each axis and ensures squareness of the first rail to the lock screw post, and establish orthogonality between axis platforms, within the limits of the small screw clearances. Screws 118 are provided, as well as pin dowels 120. Linear rail bearings 122 are provided for linear rails 124.

FIG. 20 depicts the orthopedic simulator 10 and illustrates the loading (Fz) in the z direction that is provided in the direction of arrows 128 by the vertical load actuator 26. The integral actuator 26 is integral in nature and may be a precision, seal-less actuator design in certain preferred embodiments. The piston rod is floated on an oil film, and the near zero friction maximizes the load accuracy. A low mass rod may be employed to maximize the performance of axial rotation and vertical load channels. The individual test stations 12 have their own on-off valves. A perspective view of an actuator 26 in isolation is provided in FIG. 21. A top view of the actuator 26 is depicted in FIG. 22 and a side view of the actuator 26 is depicted in FIG. 23. A cross-sectional view of the actuator 26 is depicted in FIG. 24, with an enlargement of a portion from FIG. 24 shown in FIG. 24 a.

In certain preferred embodiments, each actuator 26 has a handle 130 on the outside of the actuator 26 that operates a built-in hydraulic valve that allows a user to shut off any station individually. Hence, if a user desires to operate with fewer than six test specimens, or a specimen fails midway through the testing process and it is therefore desirable to remove that specimen from the remainder of the test cycles, the individual test station 12 may be turned off separately from the other test stations 12 without stopping the operation of the machine 10 and the testing of the other specimens. As best seen in FIG. 24, the actuator 26 includes a piston 132 that may be moved axially and rotated. The hydraulic actuator 26 includes a bottom end cap 134 and a top end cap 136. The hydrostatic bearings 138 and 140 are provided. Thrust bearings 142 provide support for a test station 12 when the device is shut off. In such a case, a test station can be removed and the machine operated without the non-operation test station 12 influencing the other test stations 12.

Pressure to extend the piston 132 along the z-axis is provided at port 144, while pressure to retract the piston 132 is provided at port 146.

The hydraulic pressure in return ports 144, 146 are connected to and fed from the central manifold 92 (see FIG. 15) in preferred embodiments. The hydraulic actuator 26 is hydrostatic and is completely without seals, including high-pressure piston seals. The hydrostatic bearings “float” the piston rod and also provide some over-turning moment capabilities. The unique design produces an actuator without seal drag (as in a typical hydraulic actuator), resulting in a device that has extremely low linear and torsional friction. The only friction is the friction that is produced from viscous oil shear. With this design, an equal Fz force is provided across all seven actuators.

Thrust bearings are provided in the end of each end cap 134, 136. The upper end cap 136 has thrust bearings lubricated by a blow-by actuator rod oil leakage. If one specimen should fail before others, an operator can turn off the station 12. The actuator 26 retracts and the assembly will ride on the thrust bearings for a continued Mz motion. The Mz motion is common for all six Fz actuators 26 at the six test stations 12. The seventh test station 14, which operates as a load and soak station for control purposes, is not connected to the Mz drive apparatus.

The central manifold 92, depicted, for example, in FIG. 26, provides an integral manifold for multiple connections and fluid tubing for the orthopedic simulator. The use of a central integral manifold greatly reduces plumbing, provides a performance improvement since there is a greater balancing of fluid and less plumbing is required, a size reduction, a cost reduction and also serves as a structural element. In other words, the central manifold 92 provides a strong cross-brace for the orthopedic simulator 10. Examples of the plumbing include providing the fluid to the extension and retraction fluid connections of the vertical load actuators 26. The central manifold 92 also provides for lubrication fluid circulation.

FIG. 25 shows the orthopedic simulator 10 and highlights the axial rotation linkage 30 originally shown in FIG. 1. The axial rotation linkage 30 provides a moment Mz at the test specimen. Referring now to FIG. 26, which shows a rear view of the orthopedic simulator 10, a linear actuator 150, via connecting link 152, provides the driving force that causes the axial rotation linkages 30 to rotate around the z-axis.

It is desirable to provide a transmission of drive torque with little deflection related error, having high torsional stiffness. At the same time, low axial stiffness is desirable so that there is little cross-talk onto the vertical loading end and so that cross-talk is not seen at the load cell. The axial rotation linkage includes a rotational transfer link 154 that is coupled to the connecting link 152. Movement of the connecting link 152 in a linear fashion causes the rotational transfer link 154 to freely rotate on bearings around the z-axis. A flexure assembly 156 that is torsionally stiff but axially compliant is coupled to the bottom of the piston 132 of the vertical load actuator 26. The flexure assembly is torsionally stiff so as to rigidly transfer torque between the rotational transfer link 154 and the piston 132 of the actuator 26. A friction free axial/torsion actuation is provided by the combination of the actual rotation linkage 30 and the friction-free vertical force actuator 26. In operation, the vertical load actuator 26 applies a load to the test specimen 40 along the z-axis by moving the piston 132 along the z-axis. Driven by linear actuator 150 through the connecting link 152, the rotational transfer link 154 and the flexure assembly 156 facilitate rigid torque transfer to the piston 132 to the test specimen (not shown) at the test station 12. The piston 132 is allowed to translate along its axis freely due to the high axial compliance provided by the flexure assembly 56 of the axial rotation linkage 30.

FIGS. 27-29 depict linkage approaches and highlight the differences between embodiments of the present invention and alternate linkage approaches which provide greater joint serialization error. In FIG. 27, a common sublinkage is provided for the flexion/extension (My) and axial rotation (Mz) to thereby create the fewest common number of joints between each specimen, between the displacement measuring device and each specimen, and between the drive actuator and each specimen. In this manner, variability is minimized. The approach provided in the present invention is depicted in FIG. 27. As can be seen, the solid cross-piece 160 provides force to all the linkages 162 at once, from the actuation mechanism 164. In the illustrated exemplary embodiments, the connecting link 24 forms the solid cross-piece 160 for connection to the gimbals 24 to provide the flexion/extension motion (My) with few joints. Similarly, the connecting link 152 forms a solid cross-piece 160 for the axial rotation (Mz). By contrast, FIG. 28 employs three separate connecting bars 166 which are connected by two links 168. Hence, those test specimens at the left side of FIG. 28 have a larger number of joints (8) than the number of joints (4) for the left-most specimen in FIG. 27. This increases the variability in the forces and motions applied to the test specimens from test station 12 to test station 12. A similar variability is provided in FIG. 29, in which a large number of joints are provided for the various test stations, with each test station having a different number of joints. Hence, the arrangement of the present invention reduces variability in force and motion application from station 12 to station 12.

FIG. 30 schematically depicts the nesting order of forces in accordance with embodiments of the present invention. This nesting order of forces is achieved by the arrangement of the linkages as depicted in the figures throughout this application.

The mechanism system generates relative motions and forces between the lower (inferior) and upper (superior) portions of orthopedic devices, such as multiple intervertebral disc implants, simultaneously to generate wear on the artificial bearing surfaces over similar motion and force induced degradation with time. The mechanism applies these motions and forces in such a way as to maximize the accuracy, test speed and durability of the linkage. The full six degree of freedom linkage system is nested as shown in FIG. 30 to maximize performance and accuracy. Typical spinal implant tests in conventional systems require higher displacements in the flexion/extension direction (My), as compared to the lateral bending (Mx) and axial rotation (Mz) rotations. These motions are often performed at a common or similar frequency and wave shapes. Therefore, the flexion/extension motion represents the most demanding performance. The mechanism system of the present invention is nested, however, so as to place the sub-mechanism with the highest required performance closest to the specimen. This thereby minimizes the moving mass and any related inertial induced error. Hence, as seen in FIG. 30, the schematically indicated specimen is indicated by reference numeral 170. The closest sub-mechanism to the superior (upper) portion of the test specimen 170 is the flexion/extension (My) sub-mechanism. The lateral bending (Mx) is further from the superior portion of the specimen 170, as indicated by FIG. 30. Finally, the drive for the Mx and My forces is furthest away from the specimen 170.

For the illustrated exemplary embodiments, the connecting link 20 b coupled to the gimbals 24 form the sub-mechanism that is closest to the superior portion of the test specimen 170. It is the flexion/extension sub-mechanism that has the highest required performance of the sub-mechanisms. The moving cross-head 32 represents the lateral bending sub-mechanism (Mx), in the illustrated embodiments. As can be appreciated from FIG. 15, for example, the moving cross-head 32 is located physically further from the test specimens than the gimbals 24 and the connecting link 20 b, as the connecting link 20 b is connected to the cross-head 32 by the linear actuator 20 a. The drive for the Mx and My motions, is represented by linkage 22 and 20 and is still further from the test specimen 170.

For the lower (or inferior) portion of the specimen 170, the forces Fy in the y direction are free, fixed or biased and have a minimized moving mass and have the highest required performance. The forces in the x direction Fx are then nested further from the specimen 170 than the Fy force. The vertical force provided by the actuator 26, Fz, is still further from the inferior portion of the test specimen 170, with the moment around the z-axis, Mz, being provided in a nesting arrangement still further from the test specimen 170. The drive for all these forces is provided as indicated. In the illustrated exemplary embodiment, the Fx and Fy motions and forces take place in the x-y slide table 100. when in the spring-loaded configuration described above. An adjustment system allows an operator to set the amount of force in each of the x and y axes. This is not a controlled degree of freedom, but rather, there is free translation if an external force overcomes the spring setting. For spinal implants that are simple ball-in-socket joints located coincident with the Mx, My and Mz centers of the machine, the spring is not engaged. However, some specimens would generate crosstalk loading into the Fx/Fy or Fz axes. This spring constraining force allows a user to simulate the soft tissue surrounding a specimen, or intentionally sideload an implant to simulate mis-implantation.

Located further away from the inferior portion of the test specimen is the vertical load actuator 26 that applies the Fz forces, in the illustrated exemplary embodiments. Still further away is the sub-mechanism for the Mz motion, which includes the connecting link 152 and the rotational transfer link 154. The drive for the Mz sub-mechanism (the axial rotation sub-mechanism) includes the linear actuator 150 and is located further from the test specimen.

Although FIG. 30 depicts a nesting order in accordance with embodiments of the invention, it is to be understood that this is nesting order is exemplary only, as other nesting orders can be used without departing from the scope of the present invention. For example, the Fx and Fy forces may be interchanged in the nesting order, assuming that the Fx force requires higher performance.

The Euler sequence of rotational motion as applied by the mechanism of the present invention is flexion/extension->lateral bending->axial rotation. In the field of testing of spinal implants, this ordering of the mechanism promotes maximum performance and minimizes the additive joint error. The independency of linkages reduces or eliminates cross-talk and allows accurate control of the phases between the individual mechanisms. This is important to create the desired and controlled loading of the test specimen 170.

FIG. 31 shows the required forces for applying to a test specimen of a spinal implant intended for the lumbar region according to the an exemplary set of curves. Similarly, FIG. 32 shows the same information for cervical data. Duty cycle loading involves inserting high loads and displacement activity into a more typical repeating activity, such as lifting a heavy box periodically. This allows for the insertion of periodic overload states. Such overload states are known to potentially induce damage, but are relatively rare so that their rarity should be considered and the overload states placed in the context of other daily activity when included. In addition to duty cycle loading, embodiments of the present invention provide for re-creating any sinusoidal or non-sinusoidal curve, which allows for more accurate simulation (e.g., a “walking simulation”). The embodiments of the invention allow for inputting non-sinusoidal data with varying phase, amplitude and frequency content, such as real walking profiles. These curves, such as shown in FIG. 33, can be repeated for a large number of cycles, and hence are fatigue or wear generating. The representation of activity is not limited to walking, as one of ordinary skill in the art will readily appreciate, but may be used to simulate any number of replicated activities in a serial or repetitive fashion. Accordingly, a controller 200, seen only in FIG. 1, is used to independently and individually control each of the motion devices. Hence, the flexion/extension, lateral bending, rotation, and loading of the test specimen 170 may be controlled to any desirable curve through the use of control software and the mechanisms provided in the orthopedic simulator 10. This allows for the testing of an orthopedic device that simulates actual conditions that the orthopedic device will be subjected to rather than the constant forces depicted in FIGS. 31 and 32 applied over 10 million cycles. For example, a test may account for the typical day for humans. Such a day may include sitting for hours at a time with intermittent periods of activity, including walking and sleeping periods. Strenuous physical activity, such as for athletes, may also be better modeled. The controller 200 thereby more accurately causes the orthopedic simulator 10 to simulate the forces that a spinal implant or other orthopedic device will actually be expected to see for a typical implant recipient.

FIG. 34 depicts the orthopedic simulator 10 within a housing 178. The use of a housing 178 prevents contamination and reduces oil within the environment. Switches 180 allow a test station to be shut down very quickly in order to prevent invalidating of a test if an individual test station 12 should experience difficulty in operation.

The embodiments of the present invention provide systems and test machines with actuators that provide loading of test specimens that is consistent across a plurality of test stations, and is accurate as the forces that are to be applied to the test specimens are not substantially affected by friction effects in the actuators.

Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. 

1. A testing apparatus for applying loads to a test specimen, the testing apparatus comprising: a frame; an actuator assembly coupled to the frame, the actuator assembly comprising: a housing; a piston axially slidable within the housing; at least one hydrostatic bearing that floats the piston relative to the housing such that the piston is movable between a retracted position and an extended position relative to the housing; and hydraulic fluid ports in the piston housing, with hydraulic fluid pressure in the hydraulic fluid ports controlling extension and retraction of the piston with respect to the housing; a first test specimen holder; and a second test specimen holder operably coupled to the frame and the first specimen holder being operably coupled to the piston to move the first specimen holder towards and away from the second specimen holder via the extension or retraction of the piston with respect to the housing.
 2. The test apparatus of claim 1, wherein the apparatus comprises a plurality of hydrostatic bearings including a first hydrostatic bearing spaced-apart axially along a direction of movement of the piston from a second hydrostatic bearing.
 3. The test apparatus of claim 1, wherein the piston is rotatable within the housing about a central longitudinal axis of the piston to supply a rotational load to the test specimen.
 4. The test apparatus of claim 1, wherein the actuator assembly includes a hydraulic shut-off valve that shuts off hydraulic fluid pressure to the hydraulic fluid ports.
 5. The test apparatus of claim 1, further comprising thrust bearings at an axial end of the housing radially spaced from the piston and configured to supply an axial load.
 6. The test apparatus of claim 5 wherein the piston provides the loads to the test specimen coupled to the first and second specimen holders through one or more components and the thrust bearings are disposed between the axial end of the housing and the one or more components.
 7. The testing apparatus of claim 1 comprising a plurality of test stations wherein each of the plurality of test stations comprise at least one actuator assembly coupled to at least one of the first or second specimen holders of each of the plurality of test stations wherein the at least one actuator assembly is configured to apply one or more loads to a corresponding test specimen at a corresponding test station and the at least one actuator assembly of each of the plurality of test stations includes the piston axially slidable within the housing and the at least one hydrostatic bearing that floats the piston relative to the housing.
 8. The testing apparatus of claim 7, wherein the at least one actuator assembly of the plurality of test stations are simultaneously driven by the hydraulic fluid pressure through a central manifold to provide generally equal loads to a plurality of test specimens at the plurality of test stations.
 9. The testing apparatus of claim 7, wherein each of the at least one actuator assembly of each of the plurality of test stations has an individual shut-off valve to turn off each actuator assembly separately from one or more other actuator assemblies.
 10. A test machine, comprising: first and second test stations each including a specimen holder assembly to hold first and second test specimens; a plurality of actuator assemblies for the first and second test stations configured to apply vertical load forces to the first and second test specimens and rotational load forces to the first and second test specimens through a plurality of pistons each movable along a longitudinal axis relative to one or more housings of the plurality of actuator assemblies and rotatable about the longitudinal axis; and an actuator coupled to the plurality of pistons through a linkage assembly to rotate the plurality of pistons to provide the rotational load forces to the first and second test specimens.
 11. The machine of claim 10, wherein the plurality of pistons of plurality of actuator assemblies are commonly hydraulically driven along the longitudinal axis through a central manifold.
 12. The test machine of claim 10 wherein the plurality of actuator assemblies each include a separate housing and each of the plurality of pistons is axially and rotationally movable within the separate housing.
 13. The test machine of claim 10 wherein each of the first and second test stations includes one or more hydrostatic bearings disposed between the pistons of each of the plurality of actuator assemblies and the one or more housings of the plurality of actuator assemblies.
 14. A test machine, comprising a plurality of test stations each including a specimen holder assembly to hold a test specimen in position for application of load forces on the test specimen; a plurality of actuator assemblies positioned relative to the plurality of test stations to apply vertical load forces to the test specimens and rotational load forces to the test specimens through pistons that are movable along a longitudinal axis wherein the plurality of actuator assemblies include a plurality of housings having one of the pistons moveable therein and each of the plurality of housings has a device to turn off one of the plurality of actuator assemblies separately from one or more other actuator assemblies.
 15. A test apparatus comprising: a first specimen holder; a second specimen holder; an actuator assembly comprising a housing; a piston disposed within the housing and the first specimen holder being operable coupled to the piston; one or more hydrostatic bearing disposed between the piston and the housing; a first actuator assembly configured to actuate the piston along a longitudinal axis to move the first specimen holder toward or away from the second specimen holder; and a second actuator assembly configured to rotate the piston about the longitudinal axis to rotate the first specimen holder relative to the second specimen holder.
 16. The test apparatus of claim 15 including a plurality of test stations and comprising a hydraulic assembly that supplies fluid pressure to a plurality of first actuator assemblies through a central manifold to move the first specimen holder toward or away from the second specimen holder.
 17. The test apparatus of claim 15 including a plurality of test stations and comprising a linear actuator coupled to a plurality of pistons through a linkage assembly to rotate the plurality of pistons to rotate a plurality of first specimen holders relative to a plurality of second specimen holders.
 18. The test apparatus of claim 15 wherein the first specimen holder is coupled to the piston through a slide table to provide relative movement between the first specimen holder and the piston.
 19. The test apparatus of claim 18 wherein the piston is configured to impart force along a first axis and the slide table provides the relative movement between the first specimen holder and the piston along a second axis different from the first axis.
 20. The test apparatus of claim 19 wherein the first axis is a z-axis and the slide table provides the relative movement along x and y axes. 